resetenv doesn't work for you because it resets to a condition whereby it tries to boot from your network. (Actually, probably not even *your* network - see
serverip=10.4.50.5).
The installer certainly wrote data to the plug; environment variables "recover1" etc demonstrate this.
The message about failing to read from USB can occur because your USB device is not properly formatted. Try FAT or FAT32, and preferably try a different USB stick too, if you can.
But finally, or firstly, and perhaps most importantly:
1. Sheevaplug at power on always went to Marvell>> prompt.
What kind of plug did you buy (ex-factory, or multiboot), and did it ever boot fully?